Why was Asiana jet flying so slowly before crash?

The Boeing airliner that crashed at San Francisco airport on 6 July – killing two teenage passengers and injuring scores more – was flying far too slowly to maintain the lift it needed to stay airborne. So says the US National Transportation Safety Board (NTSB) after an initial analysis of black box data from the Asiana Boeing 777.

The airliner's crew seemed to realise too late that they were dropping from the sky on landing approach and an attempt to boost engine power came too late to save the aircraft from striking the ground prematurely. Amateur video showed the airliner approached the runway with its nose too high and its tail too low – a sign of imminent stall – causing the tail to make contact with the ground short of the runway. The impact severed the tail section, both engines, and the landing gear.

The broken fuselage then cartwheeled, one wing twisting high in the air, before coming to a stop amid a fuel fire. Modern fire-retardant insulation and crash-resistant seating did their job, allowing passengers to be evacuated before fire consumed the wreckage. But two 16-year-old passengers ejected from the plane died.

Basic element

Because maintaining appropriate airspeed is one of the most basic elements of flying, the NTSB's initial finding is likely to involve scrutiny of both pilot training and the levels of automation in modern cockpits, which some believe can make pilots too reliant on technology.

In February 2009, failure to maintain airspeed led to a stall of Colgan Air flight 3407, which crashed in Buffalo, New York, killing 50 people. And in June 2009 Air France 447 plunged into the Atlantic Ocean after losing airspeed and stalling, killing 228 people.

All aircraft have their own ideal landing speed which, with the lift-enhancing flaps extended, allows for a smooth, controlled descent. But as Asiana flight 214 began its final approach to San Francisco's runway 28L on Saturday, says NTSB chief Deborah Hersman, its airspeed slowed below the 137 knots needed for a Boeing 777-200ER to make a safe landing. Data from the flight recorder indicate that the aircraft reached a low of 103 knots 3 seconds before impact. At that point the engines were at 50 per cent power and increasing, as the crew were throttling up to try to gain speed.

No crew member called to increase speed, she says, until 7 seconds before impact – suggesting the pilots were unaware of their plight until very late on. Three seconds later the 'stick shaker' rattled the pilot's control yoke to warn that more speed was needed to avoid a stall. At 1.5 seconds before impact they decide to go to full power and circle the airport to try landing again - but by then a crash was unavoidable.

Odd descent

It is too soon to say exactly what the precise cause was. However, the pilots had to land the plane without the help of a navigation system that can guide planes on to a glide path to the runway, as the equipment is currently out of service at San Francisco airport. The crash investigation will probably focus on to what degree the lack of that common facility was a factor in the incident. In addition, the odd descent profile of the flight – it plummeted rather quickly at times on its way into San Francisco airspace – may hold clues, too.

"I'm sure training and cockpit automation will be at the top of the things the NTSB is looking at," says Emily McGee of the Flight Safety Foundation in Alexandria, Virginia, which campaigns on behalf of passenger groups. "But they will be looking at that amongst everything else on what is a very long list."

Rob Hunter, head of flight safety at the British Airline Pilot's Association, a London based aviator's advocacy group, says the NTSB investigation is likely to focus in part on ways in which pilots can be made unaware of their speed by the complexities of cockpit computers.

"It is not uncommon for there to be events where the aircraft gets very slow and the crew don't recognise it. That can be caused by some element of the autopilot failing or misleading the crew," he says. "There are many modes and sub-modes of autopilot operation and each of these has certain protection functions which prevent the aircraft from, for instance, stalling. But in certain autopilot modes that protection is disabled. It's complicated and not particularly intuitive for pilots to understand."

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Navigation systems that would have helped guide the plane down were out of service at San Francisco airport (Image: Ezra Shaw/Getty Images)